Powder feed hoppers are commonly employed in electrostatic powder coating systems to supply air-entrained particulate powder material to spray guns located in a powder spray booth which are effective to apply a powder coating to various articles conveyed through the booth. Powder feed hoppers of this type comprise a housing having a hollow interior within which a porous, fluidizing plate is mounted near the bottom end. This porous plate is adapted to support particulate powder material which becomes "fluidized", i.e., entrained in air, by an upward flow of pressurized air through the porous plate. The upward flow of pressurized air forms a fluidized bed or cloud of air-entrained particulate powder material within the interior of the housing which is removed therefrom and transmitted to the powder spray guns by one or more powder pumps associated with the powder feed hopper.
In order to obtain an acceptable coating of particulate powder material on an article to be coated, air-entrained particulate powder material having a uniform density and an evenly mixed particle size distribution must be supplied to the powder spray guns. The term "density" as used herein refers to the relative mixture or ratio of powder-to-air, and the term "particle distribution" refers to the dispersion of powder particles of different sizes within the fluidized bed of the powder feed hopper. A number of problems inherent in the design of many powder feed hoppers have produced non-uniform density and/or uneven powder distribution within the fluidized bed, which, in turn, results in less than desirable coatings on target articles.
One problem with many powder feed hoppers is the development of stable, "chimney"-shaped channels of air within the powder material atop the porous, fluidizing plate. Once these chimneys or cylinders form, the air flow through the porous plate is easier through such chimneys than through the surrounding areas in the plate covered with powder. As a result, fluidization of the powder material carried on the porous plate is reduced or stopped and the air-entrained particulate powder cloud within the fluidized bed above the porous plate becomes substantially non-uniform in density and particle distribution.
Another problem with many powder feed hoppers is the creation of stratification of particles within the fluidized bed. Stratification is obtained by developing an upward flow of pressurized air through the porous plate which is substantially uniform in velocity across the surface area of the plate. Such velocity distribution causes the larger diameter powder particles to collect near the bottom of the fluidized bed, adjacent the porous plate, and progressively smaller diameter particles are concentrated at increasing distances from the porous plate within the fluidized bed. When the air-entrained particulate powder material is withdrawn from a stratified fluidized bed, such stream does not contain a typical distribution of particle sizes, thus producing a poor quality coating on an article to be coated.
Efforts have been made to reduce the formation of channels or cylinders of powder at the porous plate and stratification of particles according to size within the fluidized bed of powder feed hoppers, but solutions such as air distribution channels, manifolds and other constructions have proven to be relatively expensive, difficult to convert from one type of powder material to another and often result in the formation of localized "hot spots" or areas of relatively high air velocity through the porous fluidizing plate. Hot spots are a particular problem because excessive, high velocity air streams carry with them "fines" or small particles having a size of less than about 15 microns which preferably should remain within the hopper. Additionally, these hot spots or areas of high velocity air can create a "boiling" action, i.e., the formation of air bubbles within the particulate powder material, and these bubbles produce localized, non-homogeneous areas of fluidization which result in an unsteady flow of air-entrained particulate powder material to the powder pumps.